Institut fr Geowissenschaften Sektion 3.2 Deformation und Rheologie des GFZ Potsdam

Fracture Toughness Determination and Micromechanics of Rock Under Mode I and Mode II Loading

Dissertation

zur Erlangung des akademischen Grades

"doctor rerum naturalium"

(Dr. rer. nat.)

in der Wissenschaftsdisziplin " Geologie "

eingereicht an der

Mathematisch-Naturwissenschaftlichen Fakultt

der Universitt Potsdam

von

Tobias Backers

Potsdam, den 12. August 2004

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Doctoral Thesis by

Tobias Backers

Supervised by Prof. Dr. rer. nat. Georg Dresen and Prof. Ove Stephansson, PhD.

Submitted to University of Potsdam, Germany

2004 08 12

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A pdf-version of the thesis is available through the author.

Set in 9/10/11/12/14 pt Garamond.

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This thesis is dedicated to Yvonne Backers. Much too young my mother died.

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ACKNOWLEDGEMENTS/CREDITS

I debt my gratitude to a lot of people who helped me to pass the hassles of this thesis. This includes people at the GeoForschungsZentrum (GFZ) Potsdam, Germany, at the Royal Institute of Technology (KTH) in Stockholm, Sweden, and at the Ruhr-Universitt Bochum, Germany, but also my friends and family.

I acknowledge the financial support of this work through GFZ and DFG grant DR 213/9-1 and DR 213/9-3.

I want to thank my supervisors, Prof. Dr. rer. nat. Georg Dresen and Prof. Dr. Ove Stephansson for their valuable advice and guidance through my studies. Numerous and intense discussions improved my understanding of fracture mechanics and rock mechanics both from a microstructural and engineering point of view. Besides this academic support there was also room for friendly chats.

The collaboration I had with my co-authors on the publications I want to acknowledge. These are Prof. Dr.-Ing. Michael Alber, MSc (Ruhr-Universitt Bochum), Dr. Nader Fardin (KTH Stockholm), Dr. Erik Rybacki (GFZ Potsdam) and Dr. Sergei Stanchits (GFZ Potsdam). It was a very fruitful work that led to good results.

I had valuable discussions with Michael Alber on very principle things that led to satisfying experiments. Further, he let me use his facilities at Bochum University, i.e. a large diameter Hoek-Cell, the loading frame and the sample preparation laboratories. Nader Fardin did surface roughness measurements of samples. Sergei Stanchits did recording of Acoustic Emissions and analysis of the data.

Many others helped at the different research institutes whom I cannot all mention here. Also the reviewers of the manuscripts and guests at GFZ provided good discussions. Nevertheless I want to allude Stefan Gehrman (GFZ Potsdam), who helped me with sample preparation, Dipl.-Ing. Michael Naumann (GFZ Potsdam), whose ideas led to the final design of the devices for testing, and Erik Rybacki, who was always available for a discussion or chat. Thanks also to all my former and recent colleagues at GFZ, Section 3.2.

I would like to express my sincere appreciation to those who read through the thesis or parts of it and gave their comments, namely my supervisors, Ann, Anna, Ann-Elen, Erik, Geoff, Stefan and my father.

I want to thank all those who went for a beer and laughter during the course of the last three years. For mental support I want to thank my friends, and my sister, Gabi. For lots of things, which are too many to list here, thanks to Daniela.

Tobias

Potsdam, August 2004

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CONTENT

Acknowledgements VII Content IX Summary/Zusammenfassung XIII

1 INTRODUCTION____________________________________________ 1

2 INTRODUCTION TO THEORY OF FRACTURE MECHANICS AND FRACTURE TOUGHNESS DETERMINATION ______________ 5

2.1 Discontinuities in rock 5 2.2 Mode of fracturing, stress distribution, stress intensity factor and

fracture toughness 6 2.3 The Griffith concept and Energy Release Rate 8 2.4 The process of fracturing and fracture process zone (FPZ) models 9 2.4.1 The process of fracturing 9 2.4.2 Static dynamic versus stable ( subcritical) critical unstable

fracture growth 10 2.4.3 Fracture process zone models 11 2.5 Fracture toughness testing methods, influencing factors and data 13 2.5.1 Mode I fracture toughness testing methods 13 2.5.2 Mode II fracture toughness testing methods 14 2.5.3 Factors influencing fracture toughness 15 Confining pressure 15 Other parameters 16 2.5.4 Typical data on KIC and KIIC for rocks 16

3 EQUIPMENT AND MATERIALS ______________________________ 17

3.1 Loading equipment 17 3.2 Acoustic Emission equipment 17 3.3 Tested materials 17 sp Diorite 17 Aue Granite 19 Mizunami Granite 19 Carrara Marble 19 Flechtingen Sandstone 19 Rdersdorf Limestone 19

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4 MODE I LOADING METHODS, RESULTS AND DISCUSSION ___ 21

4.1 Methods 21 4.1.1 Chevron Bend method 21 Sample preparation and testing set-up 21 Testing procedure 22 Evaluation 22 4.1.2 Roughness determination 23 4.1.3 Microstructural analysis 24 4.2 Results 24 4.2.1 KIC determined by the Chevron Bend method for several rock

types 24 4.2.2 The influence of loading rate on different parameters in Mode I

testing of Flechtingen sandstone 24 Mechanical data and Fracture Toughness 24 Fracture Roughness 26 Microstructure 26 Acoustic Emission 26 4.3 Discussion 31 4.3.1 Determined fracture toughnesses 31 4.3.2 The influence of loading rate on Mode I testing of Flechtingen

sandstone 31 Mechanical data and Fracture Toughness 31 Fracture Roughness and Microstructure 32 Acoustic Emission 32

5 MODE II LOADING METHOD, RESULTS AND DISCUSSION____ 35

5.1 Method Punch-Through Shear (PTS-) test 35 Devices 35 Sample preparation and testing set-up 37 Testing procedure 37 Evaluation (Displacement Extrapolation Technique) 38 FEM analysis of suggested geometry 40 5.2 Results from experimental testing and analysis 40 5.2.1 Results from geometry variation 40 Influence of notch depth 40 Influence of notch curvature and sample diameter 42 Influence of notch width 42 5.2.2 Influence of displacement rate 43 5.2.3 Influence of confining pressure 43 5.2.4 Cyclic loading 44 5.2.5 Fracture evolution 45 5.2.6 Influence of confining pressure on the fracture pattern of Carrara

marble 48 Macro Scale observations 48 Micro Scale observations 49 5.2.7 Results from Acoustic Emission recording 52 5.3 Discussion 52 5.3.1 Geometry variation 52 Notch depth and Sample height 52 Notch diameter 52 Notch width 53

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5.3.2 Sample geometry and testing procedure 54 Geometry 54 Application of loading and confining pressure 54 Displacement rate 55 Cyclic loading/Displacement control 55 5.3.3 Evaluation method 56 Displacement Evaluation Technique (DET) 56 Box Evaluation 57 Displacement Gradient Method 57 Stress Approach 58 Energy Approaches 58 J-Integral 58 Energy Release Rate 59 Error 60 Conclusion 61 5.3.4 Fractography 61 5.3.5 Influence of confining pressure on the fracture pattern of Carrara

marble 61 Macro Scale 62 Micro Scale 62 5.3.6 Confining pressure 63

6 COMPARISON OF RESULTS OF MODE I AND MODE II LOADING AND CORRELATION ANALYSES ___________________ 67

6.1 Mode I fracture toughness correlation analyses 67 6.2 Mode II fracture toughness correlation analyses 68 6.3 Comparison of the response of rock to the applied modes of loading 70 6.4 Discussion 71

7 APPLICATION OF ROCK FRACTURE MECHANICS TO ROCK ENGINEERING PROBLEMS _________________________________ 75

7.1 Overview 75 7.2 Fracture mechanics modelling of shafts and galleries of the URL in

Mizunami, Japan 76 7.2.1 Laboratory tests of fracture toughness 76 7.2.2 Modelling of a shaft and gallery 76 Shaft 76 Gallery 77 Conclusion 77

8 GENERAL DISCUSSION_____________________________________ 79

8.1 Mode I loading 79 8.2 What is the fracture toughness of rock? 79 8.3 Mode II loading 81 Mode II fracture toughness determination 81 Microstructural breakdown process 82 Correlation analysis 82 Application 82 The status of the Punch Through Shear test 83

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9 CONCLUSIONS & OUTLOOK ________________________________ 85

9.1 Conclusions 85 9.1.1 Mode I 85 9.1.2 Mode II 85 9.2 Suggestions for Further Research 86 Mode I 86 Mode II 86 9.3 Outlook 87

10 REFERENCES _____________________________________________ 89

APPENDICES ______________________________________________ 95

A Publications i

B Specimen register and Test results iii

C Technical Drawings xi

D Template listings xvii

E Displacement Extrapolation Technique Reference Plots xxiii

Notations and Abbreviations

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SUMMARY

This thesis work describes a new experimental method for the determination of Mode II (shear) fracture toughness, KIIC, of rock and compares the outcome to results from Mode I (tensile) fracture toughness, KIC, testing using the International Society of Rock Mechanics (ISRM) Chevron-Bend (CB-) method. The fracture toughness describes the resistance of rock to fracturing. This parameter is therefore important when estimating the failure of rock and rock structures using rock fracture mechanics principles.

Critical Mode I fracture growth at ambient conditions was studied by carrying out a series of experiments on a clay bearing sandstone at different loading rates, i.e. clip-gage opening rates of 510-6 m/s to 510-10 m/s. The range of loading rates provides macroscopic fracture velocities that have been shown to cause time-dependent fracture growth in other test set-ups. The mechanical data shows that time- and loading rate dependent crack growth occurs i